1. Field of the Disclosure
The disclosure relates to a communication system. More particularly, the disclosure relates to a current-mode analog baseband apparatus.
2. Description of Related Art
In an ultra-wide band (UWB) wireless communication system, signal data is transmitted by orthogonal frequency division multiplexing (OFDM). To decode the signal data at a transceiver end, an analog baseband apparatus demultiplies the signal from radio frequency (RF) to baseband (0-250 MHz), and an analog to digital converter (ADC) is used to convert a baseband signal into a digital signal. During the above operations, a gain of the baseband must be consistent. However, in a present circuit design, a voltage-mode amplifier and a voltage-mode filter cannot be operated over 100 MHz. In a receiver of the UWB system, since the circuit is usually operated in a non-linear zone due to interference signals during a process of demultiplying the signal from the RF to the baseband, it is important to consider linearity. Moreover, with development of the semiconductor fabrication process, a circuit operation voltage becomes lower. Therefore, it is practical and necessary to design an analog baseband apparatus having features of rejecting interference signals, high linearity, and low operation voltage.
In a conventional intermediate frequency (IF) down-conversion design, design difficulties and power consumptions of a variable-gain amplifier and the voltage-mode filter are increased as bandwidths thereof are increased. Although the IF down-conversion can resolve a problem of direct current (DC) offset, other problems are encountered, for example, linearity and power consumption. Therefore, a direction-converter is still widely used in the UWB system. Anyway, in the present UWB system, the voltage-mode amplifier is used to implement the variable-gain amplifier. An input impedance of the voltage-mode amplifier is the greater the better (which preferably approaches infinity), and an output impedance thereof is the smaller the better (which preferably approaches 0). Contrary to the voltage-mode amplifier, an input impedance of a current-mode amplifier is the smaller the better (which preferably approaches 0), and an output impedance thereof is the greater the better (which preferably approaches infinity). In the present UWB system, the current-mode amplifier is not yet used to implement the variable-gain amplifier.